Geometric Dimensioning and Tolerancing (GD&T) is more than a set of symbols—it’s a language that defines how parts fit and function in the real world. When applied correctly, GD&T ensures dimensional accuracy, reduces ambiguity, and minimizes manufacturing risk. But when misused, it can lead to tolerance stack-ups, costly scrap, and assemblies that fail under load or wear prematurely.
We’ll go through each symbol in more detail, but here’s a quick look at the five most commonly misused GD&T symbols and the consequences of misapplying them to technical drawings:
GD&T is one of those areas where misunderstanding can lead to massive, costly mistakes in manufacturing and inspection. Below, we explore five symbols that engineers often misapply, why that happens, and the consequences for your product and bottom line.
Position
Position is arguably the most versatile GD&T control, yet it’s also the most misused. Many engineers treat position as a simple “location tolerance,” overlooking its true purpose: controlling both location and orientation relative to a datum reference frame.
Without a clear understanding of datums and true position, designers risk applying position incorrectly. The result? Parts that technically meet the drawing but fail in functional assembly because the orientation was never properly constrained.
Surface Profile
Profile is a powerhouse—it can simultaneously control size, form, orientation, and location. However, this flexibility often leads to misuse. Engineers sometimes apply profile without defining datums correctly or use it where a simpler control, like flatness, would suffice.
Over-specifying profile can over-constrain the part, driving up inspection complexity and cost without adding real value. The key is to reserve profile for surfaces that truly require multi-faceted control.
Concentricity*
Concentricity sounds straightforward, but its application is anything but. Engineers frequently use it when they really mean coaxiality or position.
Unlike those controls, concentricity requires measuring the median points of a feature—a process that is both time-consuming and rarely necessary. This misstep adds inspection headaches without improving functional performance. In most cases, position or runout will achieve the design intent far more efficiently.
*A note on concentricity
Concentricity is no longer a valid term under the latest ASME Y14.5 standard as of 2018, although it is still found on older drawings and some international standards. The standard eliminated it because of its confusing definition and frequent misuse, recommending that engineers instead use the position, profile, or runout controls.
Symmetry
Symmetry suffers from the same fate as concentricity: it’s often applied out of habit rather than necessity. True symmetry requires median point analysis, which is impractical for most parts.
Engineers typically intend to control balanced features, but position or profile can accomplish that goal with less complexity. Using symmetry unnecessarily inflates inspection costs and introduces ambiguity without delivering functional benefits.
Circular Runout
Runout is designed for rotating parts, yet it’s frequently misapplied to non-rotating features or misunderstood as a form control. Confusion between circular runout and total runout is common, leading to tolerance stack-up issues and unnecessary scrap.
When applied correctly, runout ensures smooth rotation and minimizes vibration. When misused, it becomes an expensive mistake that adds no real value to the design.
Most GD&T misuse stems from a single root cause: failing to define functional requirements before assigning tolerances. Engineers often overcomplicate their technical drawings by layering controls that don’t align with design intent.
The best practice? Start with the function of the part, then choose the simplest GD&T control that satisfies that intent. Doing so not only improves clarity but also reduces manufacturing and inspection costs—without sacrificing quality.
To sum up, here are the direct impacts of misusing each of the GD&T symbols discussed:
Lastly, here are some helpful GD&T-related resources:
Design for CNC Machining: GD&T
Anatomy of an Engineering Drawing